Photomask processes are used in manufacturing microelectronic devices. The photomask includes a mask pattern defining a variety of elements. The size of the mask pattern may decrease as the integration density of microelectronic devices continues to increase. Since a high quality photomask may be difficult to obtain in the case of the development of a device of less than about 100 nm, and a manufacturing cost of the photomask may increase, may be desirable to obtain the high quality photomask to reduce the photomask manufacturing cost and/or a turn around time (TAT).
As the size of the mask pattern gets so small as to reach a resolution limit of the exposure tool, it may be difficult to transfer on the electronic device substrate a pattern of the same shape and critical dimension (CD) that correspond to the mask pattern formed on the photomask due to an optical proximity effect.
Generally, since a correction process such as an optical proximity effect correction (OPC) may have a small correction range, the process may not effectively deal with a process effect of a wide range and may not deal with a process variation. It may also be difficult to estimate and correct an influence due to a pattern density or a pattern dependence when manufacturing the photomask for the development of devices of less than about 100 nm.
Also, as illustrated in FIG. 1, as a design rule of the microelectronic device reduces, a mask pattern may not be uniformly transferred according to the CD of an initially designed pattern 8 but may vary as shown at 9, depending on the position when a mask pattern 5 of a photomask 3 is transferred on a wafer 7 by illumination from an exposure light source 1 (e.g., KrF excimer laser or ArF excimer laser). In other words, deterioration in a shot uniformity on a wafer may occur.
Efforts have been made to attempt to improve the shot uniformity on a wafer. Generally, there is provided a method of correcting a CD deviation, including: manufacturing a photomask; performing an exposure on a wafer; measuring a CD to obtain a CD deviation; and etching a backside of the photomask on which light is incident. However, the above method of attempting to improve the shot uniformity on a wafer by controlling transmittance of the backside of the photomask may not solve the problem that a light intensity (or exposure dose) may vary for each position due to various optical phenomena while the light incident on the backside of the photomask passes through the photomask. Since a wide area of the backside of the photomask should be etched so that a predetermined CD deviation correction effect can be obtained, a global shot uniformity improvement for a repeated cell pattern of a DRAM may be possible but it may be difficult to improve a local shot uniformity for a core/peripheral circuit region for which the OPC is performed. To improve the local shot uniformity, an appropriate treatment of a front surface of the photomask and an automated local CD measurement may be needed.
Also, although a wide range of CD measurement data for the entire region of the photomask may be needed to obtain exact correction data, the number of measurement points may be limited practically. Also, selected measurement points may not be representative for the entire region of the photomask. Particularly, a method of automatically measuring the CD may be needed in order to obtain exact information for the local CD of the core/periphery regions having a variety of pattern sizes and pitches, but it may be difficult to grasp the CD distribution and the tendency for the entire region of the photomask by mere measurement of the partial region of the photomask using OPC.
Aerial imaging is widely used for inspection of photomasks. As is understood by those having skill in the art, the aerial image of a mask defines the light intensity distribution at the wafer plane as produced by an exposure light source and projection optics. An inspection technology based on aerial imaging can, thus, alert an operator to defects that actually print, and allow an operator to ignore those that may be present on the mask but have no impact on the final result. Aerial imaging for photomask inspection are described, for example, in the publications to Hemar et al., entitled “The View from Above”, SPIE's oemagazine, February 2004, pp. 22-25; Stegeman, entitled “10 Years of Aerial Image Measurement Systems—AIMSTM™”, Future Fab International, Vol. 16, and Budd et al., entitled “Development and application of a new tool for lithographic mask evaluation, the stepper equivalent Aerial Image Measurement System, AIMS”, IBM J. Res. Develop., Vol. 41, No. 1/2, January/March 1997. pp. 119-128.